1. Field of the Invention
The invention relates generally to printed circuit board testing, and more particularly to identifying opens in parallel connections.
2. Description of Related Art
During the manufacture of printed circuit board (PCB) assemblies, testing is performed at one or more stages to ensure that the finished product functions adequately. In some manufacturing operations, PCBs are first tested before any components are mounted to them. The components may be separately tested before they are attached to the PCBs, usually by soldering. Once the components are attached, a further test may be performed to verify that the components are properly attached. Such testing includes “opens” tests in which a determination is made whether all the contact points of the components are appropriately connected to the PCB.
Detecting open solder connections to components, such as integrated circuit devices, connectors, and sockets, on printed circuit board (PCB) assemblies continues to be a major challenge on today's manufacturing floor. Conventionally, automated testing to detect open connections, sometimes called open “pins,” has been performed by connecting test probes to the PCB assembly and measuring electrical parameters between two or more test points on the PCB assembly. For example, to determine whether a pin is open, test probes may be connected such that a path between the probes should include the pin to be tested, if the pin is properly connected to the PCB. By measuring a parameter, such as the impedance of the path, a determination may be made whether the pin is open. Such an approach is sometimes called an impedance test.
An alternative approach to automated testing for open pins is called a capacitive test. Such a test may be performed by exciting a sinusoidal test signal on a circuit trace of the PCB and measuring the amount of that signal capacitively coupled to a detector plate positioned above an integrated circuit device mounted to the PCB. If a pin is connected to the trace, the test signal can propagate from the trace, through the pin to a lead frame within the integrated circuit device. From the lead frame, the test signal will capacitively couple to the detector plate. The magnitude of the detected signal can be analyzed to determine if the connection between the pin and trace is good. For example, the amplitude of the detected signal at a test frequency may be compared to a threshold value. A detected signal above the threshold may be interpreted as a good connection. Similarly, a detected signal below the threshold may indicate the pin is open.
However, when a component has multiple pins connected in parallel, conventional capacitive or impedance test techniques cannot readily identify open pins because the parallel connected pins effectively mask the open. For example, connectors and integrated circuit sockets frequently have multiple power pins or ground pins connected in parallel. Even if one of these parallel power or ground pins is open, the remaining parallel connected pins couple the test signal to the component, allowing an ample signal to reach the detector plate. The detected signal in a capacitive test has an amplitude that cannot reliably be distinguished from the magnitude of the signal that would be expected if all of the pins were properly connected. A similar problem occurs with impedance tests. Even if some parallel connected pins are opens, the effect on measured impedance is not large enough to reliably detect, thereby masking the presence of the open pin.
Though test techniques are known for detection of open pins in parallel with other pins, these techniques have generally required extensive probing of an assembly under test. The test techniques sometimes required active measures to isolate sections of a PCB, which could be slow and could entail risk of damage to components on the PCB. Because the impact of an open pin when there are other pins connected in parallel was regarded as relatively minor, and because of the drawbacks of these test techniques, such techniques have not been widely applied.